The general art of utilizing a metal slug or billet and through various extrusion, cooling and heating steps forming mortar shells or projectiles is relatively old in the art.
Prior art methods have resulted in the production of mortar shells having a fragmentation count of only approximately 4000 particles. The fragmentation count is performed by the military establishment at a testing facility where a mortar shell is filled with an explosive as for actual field use. The shell is exploded under controlled conditions where all of the particles are retrieved, sized and counted. Such count of 4000 particles has been the accepted standard in the production of mortar shells in the past.
The steel ordinarily used for these prior art shells has been AISI (American Iron and Steel Institute) 1340 and 1040 grades. The AISI 1340 grade has a carbon content in the range of 0.38% to 0.43%, and manganese content in the range of 1.60% to 1.90%. The AISI 1040 grade, a very inexpensive steel, has a carbon content in the range of 0.37% to 0.44%, and manganese content in the range of 0.60% to 0.90%.
The steel industry through the AISI has classified the 1340 grade of steel as an "alloy steel" because of the high content of manganese, normally over 1%. As for the 1040 grade, the AISI classifies this steel as simply "carbon steel".
Additionally, both steels, i.e. 1340 and 1040 are referred to in the industry as "hypoeutectoid steels". These are steels with carbon content below approximately 0.80%. Steels with carbon content above approximately 0.80% are referred to as "hypereutectoid steels." See "Metallurgy For Engineers", Second Edition, 1962, O. Van Nostrand Company, Inc., New Jersey for definitions of eutectoid steels.
In fabricating prior art mortar shells with the accepted standard of particle fragmentation characteristics, the practice has been to cut slugs or billets from round bar or round corner square bar AISI 1340 steel, shot blast a slug, precoat it with lubricant, heat the slug in a furnace to approximately 1500.degree. F., or 2150.degree. F., introduce it while hot into a two cavity die and punch assembly wherein the slug is converted first into a preshape in a form somewhat like a bullet, then into an extrusion having the general form of an elongated hollow cup open at one end. Numerous cold steps follow, including further drawing, machining, ironing and shaping, then stress relieving with conventional preparatory and finishing operations in order to complete the shell.
For an 81 millimeter shell, as an illustration, the AISI 1340 steel slug is approximately 3 inches in diameter, 3-11/16" long, weighing slightly over 7 lbs. This eventuates into a tapered shell about 10 inches long with a maximum O.D. slightly over 3 inches and with a wall thickness of about 0.222" throughout most of its length, with variations at both ends. The general shape is illustrated in FIG. 2. A shell thus manufactured by the above procedures has a fragmentation of about 4,000, that is, upon explosion, disintegrates into about 4,000 particles of steel.
However, with the advent of changes in warfare and terrain, the possible need for a mortar shell possessing the characteristics of a greater number of particles upon fragmentation has developed. With such an increased count, the effectiveness of the shell would be toward the wounding of a greater number of military personnel in certain types of terrain.
The major problem encountered in endeavoring to increase fragmentation while using a steel such as AISI 1340 or 1040, is that being hypoeutectoid an increased fragmentation cannot be produced without expensive hardening operations or the use of complex engraved internal patterns as mechanical aids to fragmentation.
This has been recognized by Paul J. Horvath, Jr., a leading authority in the production and uses of steel. In his U.S. Pat. No. 3,547,032 he states as follows:
"Shell bodies manufactured from hypoeutectoid steels are generally too ductile to provide good fragmentation characteristics". In the above patent, Mr. Horvath only utilizes hypereutectoid steels because all of his examples as well as his claim call for carbon over 1.00%.
Thus, heretofore, the production of a shell with increased fragmentation capabilities has been practically unobtainable when using a hypoeutectoid steel such AISI 1040 or 1340.